Techniques for transmitting/receiving data via a cellular phone have been developed in recent years so that users can enjoy rich contents (contents having a large amount of data) such as moving images and music on their cellular phones. However, radio base stations do not have enough line or channel capacity for delivering such rich contents to users' cellular phones, and telecommunications carriers cannot offer the service at low price. Besides, users still hesitate to download the rich contents since radio communication is generally charged at a metered rate (accounting is given depending on the amount of downloaded data).
As a technique for dealing with the above problems, there has been disclosed a method for delivering data to mobile terminals through broadcasting type service channels in Japanese Patent Application laid open No. 2002-26836. FIG. 1 is a diagram showing the image of a download method (broadcasting type service) according to the prior art. In the prior art, content (broadcast program) is delivered to a plurality of users via prescribed channels at the fixed hour. That is, the users cannot receive and obtain desired contents at desired time or on demand.
Incidentally, the broadcasting type service is a type of service in which data (program, contents) are unilaterally sent to users at the proscribed hour.
As another technique for dealing with the above problems, there has been disclosed a method of simultaneous data delivery to plural user terminals by sharing the same downlink channel in radio links (a transmission channel from a radio base station to the mobile terminals of users) in Japanese Patent Application laid open No. 2001-69558. According to the technique, when a user requests a download of content within a certain period of waiting time (for example, five seconds) after another user has made a request for the same content, the content is delivered simultaneously to the users via the same downlink channel. In addition, when a user (second user) requests a download of content while another user (first user) is downloading the content, the same downlink channel being used for downloading the content to the first user is also allocated for the second user, and the second user receives the content from the middle of content data. FIG. 2 is a diagram showing the image of downloading (in which the same channel is shared by two users from the middle of downloading) according to the prior art.
With this technique, however, in the case where plural user terminals share the same downlink channel to download content from the beginning of content data, such channel shared downloading cannot be performed unless a request for the same content is made within a certain period of waiting time. In the description of the above prior art, the waiting time is assumed to be five seconds as an example. When the waiting time is short, the probability of the same requests occurring in the waiting time is decreased. On the other hand, long waiting time delays the time which it takes to complete downloading after a user made a request, and the practicability of service is reduced. Besides, in the case where a user (second user) requests a download of content while another user (first user) is downloading the same content, the same downlink channel being used for downloading the content to the first user is also allocated for the second user, and the second user receives the content from the middle of content data via the downlink shared channel as described above. Consequently, user(s) other than the first user cannot obtain the entire data of the desired content.